1. Field of the Invention
The present invention relates to a bolt and, more particularly, to one adapted to ultrasonic axial tension measurement. The term "axial tension" used herein stands for a tightening force generated in the tightened bolt in its axial direction.
2. Description of the Related Art
It has been a conventional practice to conduct ultrasonic measurement of the axial tension generated in the tightened bolt in the axial direction for secure bolt tightening. The aforementioned axial tension measurement is conducted through ultrasonic radiation to one end surface of the bolt and detection of reflection (echo) from the other end surface of the bolt in the axial direction. The axial tension generated in the bolt will be measured based on the detected results. Referring to FIG. 3, the time taken from ultrasonic radiation to detection of the reflection before tightening the bolt (FIG. 3A) is shorter than that after tightening the bolt (FIG. 3B) by the time t owing to generation of the axial tension in the bolt. The time difference t will vary in proportion to the axial tension. Accordingly the time taken from ultrasonic radiation to reflection detection has been conventionally measured so as to measure the axial tension.
An axial tension measurement device is generally formed of a piezoelectric element which is mounted to a head of the bolt for ultrasonic radiation thereto and detection of reflection. This device has been applied to a bolt tightening device such as a nut runner which rotates a socket engaged with the bolt head for rotation. The bolt tightening device is driven while the axial tension generated in the bolt is measured by the axial tension measurement device. When it is detected that the axial tension reaches a target value, operation of the bolt tightening device is stopped. In this way, the bolt can be tightened such that a constant axial tension is generated.
Referring to FIG. 5, both a top surface 22a of a head portion 22 to which ultrasonic radiation is applied from a piezoelectric element 31 and a bottom surface 23a of an axial portion 23 of a generally employed bolt 21 as the other end surface thereof in an axial direction from which the ultrasonic radiation is reflected have substantially flat surfaces. As a result, each propagation path of both the ultrasonic radiation and the reflection will be diffused within the axial portion 23 (as shown by an arrow Uwa in FIG. 6). The time taken by the diffused reflection to reach the piezoelectric element 31 may vary depending on the propagation path. As a result, a plurality of different reflections are detected.
Referring to FIG. 5, the generally employed bolt 21 is formed such that a sectional area (diameter) d1 of the head portion 22 to be engaged with a socket (not shown) of the bolt tightening device is larger than the sectional area (diameter) d3 of the axial portion 23 that has been threaded. Accordingly all the ultrasonic radiation applied to the head portion 22 is not always reflected from a bottom surface 23a of the axial portion 23. That is, detected reflection may include the one reflected from the end surface 22b of the radially extended portion of the head portion 22 (as shown by an arrow Uwb in FIG. 5). Referring to FIG. 7, it is difficult to distinguish the reflection from the bottom surface 23a of the axial portion 23 from those of a plurality of detected reflections Uwa. Furthermore, the reflection Uwb from the end surface 22b of the extended portion of the head portion 22 of the bolt 21 may be a cause of noise, thus degrading accuracy of the axial tension measurement.
On the foregoing ground, publication of Japanese Utility Model No. SHO 60-194473 discloses a device for detecting axial tension of a bolt incorporating an ultrasonic transducer disposed in close proximity to the top surface of the bolt head for ultrasonic radiation into the bolt. In the aforementioned device, an ultrasonic lens for converging ultrasonic radiation from the ultrasonic transducer to the top surf ace of the bolt head is interposed therebetween. A concavity having relatively a large radius of curvature is formed in a back surface of a cap to which the ultrasonic transducer is adhered, which may form a convex-like space having one flat surface defined by the bolt head. A convex ultrasonic lens can be defined by the space filled with oil.
In the aforementioned device for detecting axial tension of the bolt, the ultrasonic lens converges the ultrasonic radiation applied from the ultrasonic transducer to the top surface of the bolt head. The ultrasonic radiation applied into the bolt head is focused to the cross section of the axial portion of the bolt as it propagates from the bolt head to the axial portion. The ultrasonic radiation applied into the bolt head is, thus, efficiently propagated within the axial portion of the bolt. The ultrasonic radiation exhibiting relatively higher intensity can be propagated within the axial portion of the bolt for detecting the ultrasonic propagation time. As a result, the signal level upon receipt of the ultrasonic radiation is raised to a higher level compared with other noise, thus detecting the axial tension of the bolt with high accuracy. The aforementioned generally employed art discloses the description with respect to change in the shape of the space constituting the ultrasonic lens for converging the ultrasonic radiation depending on the sonic speed of propagation between the substance filled in the space and the cap-forming material(ultrasonic propagation characteristics).
However, the length of the bolt subjected to the axial tension measurement is not always constant. Accordingly, in the device for detecting the axial tension of a bolt disclosed in Publication of Japanese Utility Model SHO No. 60-194473, the ultrasonic radiation applied from the ultrasonic transducer to the top surface of the bolt head can be converged by the ultrasonic lens. However, the convergence degree of the ultrasonic radiation applied from the ultrasonic transducer to the top surface of the bolt head may vary. Therefore it is necessary to select the type of the ultrasonic lens, concave or convex (the concavity formed in the back surface of the cap to which the ultrasonic transducer is adhered), lens shape including the curvature, and ultrasonic propagation characteristics such as the material with which the lens-forming space is filled in accordance with the bolt length.
In the above-identified reference, convergence of the ultrasonic radiation applied from the ultrasonic transducer to the top surface of the bolt head is considered. However, the convergence of the ultrasonic reflection from the bottom surface of the axial portion of the bolt is not considered. As the propagation path of the reflection becomes complicated, the reflection is diffused while being detected.